1. Field of the Invention
The present invention relates to a wavelength tunable light source, and more particularly, to a wavelength tunable light source that can electrically tune wavelengths by integrating an optical amplifier, a beam steering unit, and a concave diffraction grating on a single substrate.
2. Description of the Related Art
In general, the use of wavelength tunable semiconductor lasers, i.e., wavelength tunable light sources is becoming more important with the advent of optical transmission technology such as wavelength division multiplexing (WDM). Since wavelength tunable semiconductor lasers have many compelling advantages over fixed wavelength lasers that emit respective different wavelengths, the wavelength tunable semiconductor lasers can be a good substitute for the fixed wavelength lasers. Particularly, wavelength tunable semiconductor lasers are used very effectively in reconfigurable optical add/drop multiplexing (ROADM), fast packet switching over all optical networks, wavelength converting, wavelength routing, etc. Besides, those wavelength tunable semiconductor lasers are extensively used in optical sensing, medical treatment, and measurement applications. Accordingly, many of the world's leading companies have reported various types of wavelength tunable semiconductor lasers. An external-cavity wavelength tunable semiconductor laser among conventional wavelength tunable semiconductor lasers will now be explained for clear understanding of the present invention.
FIG. 1 is a schematic view of a conventional external-cavity wavelength tunable light source in Littrow configuration.
In detail, the conventional external-cavity wavelength tunable light source includes a semiconductor laser, e.g., a laser diode (LD), 14 coated with an anti-reflection film 12, an external diffraction grating 16, and a lens 18. The semiconductor laser 14 can produce a beam 20 using current ILD applied thereto. When the beam 20 produced by the semiconductor laser 14 and passing through the lens 18 reaches the external diffraction grating 16, the beam 20 is diffracted at an angle θ between the beam 20 entering the diffraction grating 16 and a line 2 perpendicular to the plane of the diffraction grating 16. According to the following diffraction grating equation 1 for the Littrow configuration, the wavelength of the diffracted beam 21 is determined, and only a beam 21 with a specific wavelength is fed back to the semiconductor laser 14 to output light Pout.mλ=2d sin θ  (1)where m is the diffraction order, λ is the wavelength, d is the period of the diffraction grating 16, and θ is the incidence angle.
When the diffraction grating 16 is rotated in a first direction 22 about a pivot point 4, which is a virtual intersection point between the extension line of a left surface of the semiconductor laser 14 and the extension line of the plane of the diffraction grating 16, the incidence angle θ is changed and the wavelength is also changed according to Equation 1. If the incidence angle θ alone is changed in the above structure, the wavelength has a step-like shift disadvantageously. Accordingly, to achieve continuous wavelength tuning, the diffraction grating 16 should be translated in a second direction 24 as well.
In other words, the conventional external-cavity wavelength tunable light source of FIG. 1 can achieve continuous wavelength tuning by changing the diffraction condition through the displacement, that is, rotation and translation, of the diffraction grating 16 about the pivot point 4. The conventional external-cavity wavelength tunable light source of FIG. 1 has the advantages of high output power, narrow linewidth, and wide tuning range, and thus is now widely used for measurement equipment.
However, the external-cavity wavelength tunable light source has problems in that it is difficult to properly align the semiconductor laser 14 and the diffraction grating 16, mechanical vibration occurs during the displacement of the diffraction grating 16, and a wavelength shift is caused by a change in the position of the pivot point 4 with aging. In particular, since the external-cavity wavelength tunable light source of FIG. 1 has a very slow wavelength tuning rate, it is difficult to be applied to optical communication systems and various application systems.
FIG. 2 is a schematic view of another conventional external-cavity wavelength tunable light source in Littman configuration.
In detail, the conventional wavelength tunable light source includes a semiconductor laser, e.g., an LD, 14 coated with an anti-reflection film 12, an external diffraction grating 16, a lens 18, and a reflective mirror 6. The semiconductor laser 14 produces a beam 20 using a current applied thereto. When the beam 20 produced by the semiconductor laser 14 and passing through the lens 16 and the diffraction grating 16 reaches the reflective mirror 6, only part 21 of the beam 20 which is perpendicularly incident on the reflective mirror 6 is reflected back to the diffraction grating 16. The reflected beam 21 passes through the diffraction grating 16 and the lens 18 and is fed back to the semiconductor laser 14 to output light Pout. The beam 20 is diffracted at an angle α between the beam incident on the diffraction grating 16 and a line 2 perpendicular to the plane of the diffraction grating 16 and the reflected beam 21 is diffracted at an angle β between the beam 20 diffracted by the external diffraction grating 16 and the perpendicular line 2 according to the following diffraction grating equation 2 for the Littman condition.mλ=d(sin α+sin β)  (2)where m is the diffraction order, λ is the wavelength, d is the period of the diffraction grating 16, α is the incidence angle, and β is the diffraction angle.
When the reflective mirror 6 is rotated in a first direction 22 about a pivot point 4, the diffraction angle β is changed while the incidence angle α is fixed, and the wavelength is also changed according to Equation 2. If the diffraction angle β alone is changed in the above structure, the wavelength has a step-like shift disadvantageously. Accordingly, in order to achieve continuous wavelength tuning, the reflective mirror 6 should be translated 24 in a second direction 24 as well.
In other words, the conventional external-cavity wavelength tunable light source of FIG. 2 can achieve continuous wavelength tuning by changing the diffraction condition through the displacement, that is, rotation and translation, of the diffraction grating 16 about the pivot point 4. The conventional external-cavity wavelength tunable light source of FIG. 2 is structurally more stable than the conventional external-cavity wavelength tunable light source of FIG. 1 since the diffraction grating 16 is fixed and only the reflective mirror 6 is moved during the wavelength tuning.
However, like the conventional wavelength tunable light source of FIG. 1, the external-cavity wavelength tunable light source of FIG. 2 has problems in that it is difficult to properly align the semiconductor laser 14 and the diffraction grating 16, mechanical vibration occurs during the displacement of the reflective mirror 6, and a wavelength shift is caused by a change in the position of the pivot point 4 with aging. Further, since the external-cavity wavelength tunable light source of FIG. 2 has a very slow wavelength tuning rate, it is difficult to be applied to optical communication systems and various application systems.
To increase the slow wavelength tuning rate of the conventional external-cavity wavelength tunable light sources of FIGS. 1 and 2, structures for electrically tuning wavelengths have been suggested. For example, in “Continuous tuning of an electrically tunable external-cavity semiconductor”, Optics Letters, Vol. 25, No. 16, pp. 1165–1167, Aug. 15, 2000, reported by M. Kourogi et al., a method of achieving wavelength tuning based on beam deflection according to the frequency of an external electrical signal by inserting an Acouto-optic modulator (AOM) between a laser diode and a diffraction grating instead of by moving the diffraction grating for wavelength tuning was suggested. However, the invention disclosed by M. Kourogi et al. has problems in that the AOM is large in size, insertion loss is huge, and a wavelength tuning variation is limited to 2 nm at most.
In short, the conventional wavelength tunable light sources for tuning the wavelength through the displacement of the diffraction grating have limitations in reliability and speed. The conventional bulk wavelength tunable light source for electrically tuning the wavelength has the problems of difficult alignment between the diffraction grating and the laser diode and large size due to the insertion of the AOM.